1. Field of the Invention
The present invention relates to a magnetoresistive effect element (MR element) for reading magnetic field intensity of a magnetic recording medium as a signal, a thin film magnetic head including the MR element, a head gimbal assembly (HGA) and a magnetic disk apparatus.
2. Description of the Related Art
Recently, in association with high recording density of a hard disk (HDD), improvement of performance of the thin film magnetic head is in demand. As the thin film magnetic head, a composite-type thin film magnetic head with a structure where a reproducing head having the MR element exclusive for reading and a recording head having an induction-type magnetic transduction element exclusive for writing are laminated is widely used.
At present, as the reproducing head, an MR element with a so-called current-in-plane structure (CIP-GMR element) that is activated by applying an electric current on a film surface of the element in parallel, referred to as a spin-valve GMR element, is widely used. The spin-valve GMR element with such structure, in the reproducing head, is positioned between upper and lower shield layers made of a soft magnetic metal film, and, is arranged in a form interposed by an insulating layer referred to as a gap layer from top and bottom. The recording density in the bit direction is determined by a space (reproducing gap space) of the upper and lower shield layers.
In association with the increase of the recording density, demands for a narrower shield gap and a narrower track in a reproducing element of the reproducing head have become stronger. Because of the narrower track of the reproducing element and shortening of the element height in association with this, an area of the element is reduced, but because a heat dissipation efficiency is decreased with the conventional structure in association with the reduction of the area, there is a problem that an operating current is restricted from a viewpoint of reliability.
In order to solve such problem, a GMR element with a Current-Perpendicular-to-Plane (CPP) structure (CPP-GMR element) where the upper and lower shield layers (upper shield layer and lower shield layer) and the MR element are electrically connected in series is proposed, and in order to accomplish the recording density exceeding 200 Gbits/in2, this is referred to as an essential technology.
Such CPP-GMR element has a lamination structure including a first ferromagnetic layer and a second ferromagnetic layer formed so as to interpose a conductive nonmagnetic intermediate layer from both sides. The lamination structure of a typical spin-valve type CPP-GMR element is a lamination structure where a lower electrode/an antiferromagnetic layer/a lower ferromagnetic layer/a conductive nonmagnetic intermediate layer/an upper ferromagnetic layer/an upper electrode are sequentially laminated from the substrate side.
The magnetization direction of the lower ferromagnetic layer, which is one of the ferromagnetic layers, is pinned so as to be perpendicular to that of the upper ferromagnetic layer when an external application magnetic field is zero. The magnetization direction of the lower ferromagnetic layer is pinned by adjoining the antiferromagnetic layer and by applying a unidirectional anisotropy energy (also referred to as “exchange bias” or “coupling magnetic field”) to the lower ferromagnetic layer due to exchange coupling between the antiferromagnetic layer and the lower ferromagnetic layer. Consequently, the lower ferromagnetic layer is also referred to as a magnetization pinned layer. In the meantime, the upper ferromagnetic layer is also referred to as a free layer. In addition, it is also proposed that the magnetization pinned layer (lower ferromagnetic layer) has a three-layer structure (so-called “synthetic ferromagnetic (SyF) structure” or “synthetic pinned structure”) with a ferromagnetic layer/a nonmagnetic metallic layer/a ferromagnetic layer, as well. With this design, strong exchange coupling is provided between two ferromagnetic layers of the magnetization pinned layer (lower ferromagnetic layer), and exchange-coupling force from the antiferromagnetic layer can be effectively increased, and in addition, it becomes possible to reduce an effect of a static magnetic field generated from the magnetization pinned layer on a free layer. Consequently, this “synthetic pinned structure” is widely used at present.
However, in order to respond to the demand for recent ultrahigh recording density, it has become necessary to further reduce the thickness of a layer of the MR element. Under such circumstances, for example, a new GMR element structure having a simple three-layer lamination structure with a ferromagnetic layer/an nonmagnetic intermediate layer/a ferromagnetic layer as disclosed, for example, in U.S. Pat. Nos. 7,019,371B2 and 7,035,062B1, as a basic structure is proposed. In this GMR element structure, as shown in FIG. 22, two ferromagnetic layers 401 and 402 are exchange-coupled so as to have their magnetizations 401a and 402a to be antiparallel to each other. Then, a permanent magnet HM is arranged at a back side position, which is opposite from ABS that is equivalent to a medium opposing surface of the element, and the magnetizations 401a and 402a of the two ferromagnetic layers 401 and 402 are inclined at approximately 45 degrees relative to a track width direction by the bias magnetic field generated from the permanent magnet HM, respectively, and an initial state where they are substantially orthogonalized is produced (see FIG. 23). When the element in this initial magnetization state detects a signal magnetic field from a medium, the magnetization directions of the two ferromagnetic layers 401 and 402 are changed as if the operation of scissors cutting paper, and as a result, a resistance value for the element is changed. Furthermore, such element structure is referred to as a dual free layer (DFL) element structure in the present specification as a matter of convenience.
When this DFL element structure is applied to a TMR element or a CPP-GMR element, it becomes possible to further narrow a “read gap”, which is a space of the upper and lower shield layers 403 and 404, compared to a general spin-valve type CPP-GMR element. Specifically, an antiferromagnetic layer, which is required for the general spin-valve type CPP-GMR element, becomes not required, and in addition, the ferromagnetic layer in “synthetic pinned structure” also becomes not required.
In order to form the DFL element structure, the two ferromagnetic layers 401 and 402 need to be exchange-coupled so as to have their magnetizations 401a and 402a to be antiparallel to each other. Such structure is easily formable by inserting metal, such as Au, Ag, Cu, Ir, Rh, Ru or Cr, between the two ferromagnetic layers 401 and 402, and by generating exchange-coupling between the two ferromagnetic layers 401 and 402.
However, in the TMR element, an insulating film, such as an aluminum oxide (AlOx) film or a magnesium oxide (MgO) film, has to be intervened between the two ferromagnetic layers in order to obtain a tunnel effect, and inconvenience that it becomes difficult to generate a strong exchange coupling between the two ferromagnetic layers can occur. As a result, it becomes extremely difficult to bring the magnetizations of the two ferromagnetic layers to an antiparallel state.
Further, in association with the recent ultrahigh recording density, it becomes essential to improve the resolution in a cross-track direction in the CPP-GMR element, but in the CPP-GMR element using the DFL element structure in the prior art, there is a problem where the resolution capacity in the cross-track direction is still insufficient.
In addition, in the head structure using the DFL element structure, in order to realize sufficient bias magnetic field intensity for forming the initial state from the permanent magnet HM, such as CoPt, arranged at the back side position opposite from the ABS, the thickness of the permanent magnet HM has to be increased. Increase in the thickness of the permanent magnet HM means that an advantage that the DFL element structure is a structure that can narrow a read gap cannot sufficiently be fulfilled. If the thickness of the permanent magnet HM is attempted to be increased and the read cap is attempted to be narrowed, the space between the permanent magnet HM and the upper and lower shield layers 403 and 404 becomes smaller, respectively, and a bias magnetic field to be generated from the permanent magnet HM passes through the upper and lower shield layers 403 and 404, and the application of the bias magnetic field to the element becomes insufficient and a problem that a resistive change of the element can no longer be detected can occur.
In addition, in the head structure using the DFL element structure, the permanent magnet HM is arranged at the back side position, which is opposite from ABS, and the initial state is attempted to be formed in the two ferromagnetic layers 401 and 402 by applying the bias magnetic field from the permanent magnet HM to the two ferromagnetic layers 401 and 402. However, the bias magnetic field from the permanent magnet HM may leak from the element, and due to the leaked magnetic field, a problem(s) that a signal is falsely written into a medium or a signal recorded in a medium demagnetizes or degausses may also occur.